You cannot create a new Master-Detail relationship on an existing custom object if records already exist. You must first create a Lookup relationship, populate the lookup field with data in all records, and then change the relationship type to Master-Detail.
To solve the problem and create Contact__c and Property__c fields you need to delete all the records in the object tab fg. Offer Tab. After that you'll able to create those fields. Window with new offer will have contact and property fields.
If you don't want to delete all your offer entries then check that link: https://help.salesforce.com/articleView?id=000005186&type=1
https://developer.salesforce.com/forums/?id=906F00000008ybkIAA
各月ごとの金額を管理したいらしい
I don't think you can do it without monthly records as detail objects.
I think there is a way to create detail objects using automated processes such as flows.
間違って作成したけど、削除できないらしい
レコードページで使っていたら消せない？
